1. Field of the Invention
The present invention relates to an apparatus and a method for measuring the mass flow rate of material flowing through at least one vibrating conduit and more specifically relates to Coriolis type mass flow meters, and, more particularly, to an improved method and circuit for processing two electrical signals obtained from motion detectors coupled to one or more tubular conduits carrying a flowing mass and driven or excited so as to experience Coriolis acceleration.
2. Brief Description of the Prior Art
Numerous techniques are known in the prior art for processing the information that can be obtained by measuring the Coriolis forces or the effects therefrom induced in a straight, bent, or looped conduit. For example, in U.S. Pat. Nos. 3,329,019, 3,355,944, and 3,485,098, the velocity of conduit displacement on opposite sides of a "drive" point is measured and the difference therebetween is read using an alternating current voltmeter calibrated to provide an indication of mass flow rate.
However, as will be pointed out below, the use of the difference between two velocity signals as a means of indicating mass flow rate involves a factor of w corresponding to the drive frequency. If the drive frequency is not constant, errors in determining mass flow rate result. Also, under resonant operation, the drive frequency w varies with fluid density, and such variations result in substantial mass flow rate measurement error. In the above-cited prior art documents, this source of error was not recognized.
Further, in the above-cited prior art documents, the driving means were controlled with a signal that was proportional to the sum of the two "velocity" signals measured.
However, it has been found in the use of similar circuits that, as circuit components age, or if such components are replaced, or the conduit driving means (e.g. magnet and/or coil) are replaced, the sum of the velocity signals will not provide for constant drive level. Thus, errors in the mass flow rate measurement will result in direct proportion to changes in drive amplitude, and flow meter recalibration will be necessary.
U.S. Pat. Nos. 4,660,421 and 4,655,089 teach the detection of non-linear variation of phase shift with mass flow rate, a technique which although effective, is undesirable from a signal processing view point. U.S. Pat. No. 4,655,089 also explains how the time difference method disclosed in U.S. Reissue Pat. No. 31,450 and in U.S. Pat. Nos. 4,422,338 and 4,491,025 fails to properly take into account problems associated with drive frequency variations. Each of the systems disclosed in the above-mentioned patents can be adversely affected by excessive vibrational noise or hydraulic noise creating spurious signals in the motion detectors, and such noise will excite unwanted resonant vibratory modes in the tubular conduits which contribute adversely to signal measurement.
In an attempt to provide an accurate measurement at or near the oscillation mid-plane, U.S. Pat. No. 4,422,338 requires the use of analog sensors which are linearly representative of the actual motion over the full range of motion, a limitation that adds both cost and complexity to the design.
U.S. Pat. Nos. 3,132,512 and 3,276,257 disclose methods of processing analog signals obtained from "velocity" pick-up devices, but require additional signal inputs obtained from a driving motor, or other velocity pick-ups, to provide reference signals for its signal processing. Besides requiring a pair of sensing coils and a pair of reference coils, U.S. Pat. Nos. 3,132,512 and 3,276,257 connect the sensory coils in series opposing fashion, a technique that requires precise matching of sensing pick-ups to obtain no flow rate signal at zero flow.